Previous tunable Raman oscillators have incorporated frequency dispersive elements such as prisms or gratings in order to separate different frequencies in space so that a desired frequency may be selected and incorporated in the feedback process required for oscillation. For example, U.S. Pat. No. 3,705,992 issued to E. P. Ippen et al on Dec. 12, 1972 discloses tuning by means of a prism (FIG. 3) which refracts different frequencies at different angles or by means of a lens with severe chromatic aberrations (FIG. 4) which focuses different frequencies at different positions along the optic axis. Both these devices are spatial dispersion tuning devices, since selection of a desired frequency is accomplished by separating frequencies in space, either in angle or position. This prior work involves the use of a short fiber (190 cm) and a laser pump pulse that is long relative to the transit time of the fiber, so that the oscillator light at the desired frequency can make many round trips in the oscillating cavity during the pump pulse.